Fully-Integrated Polarization-Diversity Coherent Receiver Module for ...

Fully-Integrated Polarization-Diversity Coherent Receiver
Module for 100G DP-QPSK
1. Introduction
A. Beling, N. Ebel, A. Matiss, and G. Unterbörsch
u2t Photonics AG, Reuchlinstr. 10/11, 10553 Berlin, Germany
Phone: ++49(0)30726113540, Fax: ++49(0)30726113800, E-mail: beling@u2t.de
M. Nölle, J. K. Fischer, J. Hilt, L. Molle, and C. Schubert
Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute, Einsteinufer 37, 10587 Berlin, Germany
F. Verluise and L. Fulop
Kylia, 10, rue de Montmorency,75003 Paris, France
Abstract: A new compact coherent receiver module comprising a micro-optic polarization beam
splitter and two InP-based 90 0 hybrids with integrated photodiodes is characterized at 112 Gbit/s
DP-QPSK. A penalty of only 1.3 dB in OSNR sensitivity at a BER of 10 -3 compared to theory was
obtained at C-band wavelengths.
©2010 Optical Society of America
OCIS codes: (060.1660) Coherent Communication; (250.3140) Integrated optoelectronic circuits
The polarization diverse dual-quadrature coherent receiver is a key component in 100G DP-QPSK long-haul fiber
optic links. In its typical configuration it consists of two polarization beam splitters, two optical 90 hybrids, eight
photodiodes (PDs) and two transimpedance amplifiers. Because of this complexity, a fully-integrated coherent
receiver in one package is highly desirable. It saves footprint, packaging cost and reduces the number of fiber-based
inter-component connections and thus allows for a better control of skew. Different degrees of receiver integration
have been published recently including a coherent detector module based on InP-monolithic integration [1], a
monolithic polarization-diversity coherent receiver photonic integrated circuit (PIC) on Si [2], a monolithic InP
multi-wavelength polarization-diversity coherent receiver PIC [3], a dual-polarization coherent receiver module [4],
and a polarization-diversity coherent receiver module with silica-based hybrid integration [5].
In this paper we report on a new compact 100G polarization-diversity coherent receiver module based on hybrid
integration of a free-space micro-optic polarization beam splitter (PBS) with InP PICs. The fully packaged receiver
is successfully operated in a back-to-back experiment with 112 Gbit/s non-return-zero (NRZ) dual-polarization
quadrature phase shift keying (DP-QPSK) signals at 1530 nm, 1550 nm and 1563 nm wavelength.
2. Polarization-diversity coherent receiver module
The main building blocks of the receiver are shown in fig. 1. The micro-optic assembly is symmetric for both signal
(SIG) and local oscillator (LO) paths and consists of a collimator (C), two half wave plates (HWP), the PBS cube, a
skew compensator (SC) and two micro-lens arrays (L). Whereas the SIG input polarization is arbitrary, the linearly
polarized LO light is launched through the slow axis of the polarization maintaining input fiber (PMF). For
convenience the first HWP is used to ensure 3 dB LO splitting ratio. However, it should be noted that the HWP can
be omitted in future assemblies once the PMF slow axis is aligned correctly with regard to the PBS. To achieve
highest symmetry in both channels, X and Y, a second HWP is integrated in the X channel to perform a TM-to-TE
SIG
LO
HWP
C HWP
PBS
SC
L
L
InP PIC
InP PIC
90° Hybrid
X channel
90° Hybrid
Y channel
Fig. 1. Coherent receiver architecture.
OML5.pdf
TIA
TIA
TIA
TIA
XI
XQ
YI
YQ
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Fig. 2. Coherent receiver module. The second fiber pipe is
not used and will be removed in a future package re-design.

conversion. In return and to minimize the channel path length difference a skew compensator (SC) is integrated in
the Y channel. Due to mechanical tolerances we estimated a channel skew of less than 5 ps. The four beams are
finally coupled through micro-lens arrays into the integrated optical spot size converters of the InP-based PICs. Each
of the two PICs comprises a 2x4 multi-mode interference (MMI) coupler 90 hybrid together with four waveguide
pin photodiodes [1]. Four linear differential transimpedance amplifiers (TIAs) are co-packed in the module [4]. The
package with a body size of 27 mm x 40 mm x 6 mm has a coplanar waveguide rf-interface for surface mount
assembly of the device (fig.2). The 3 dB bandwidth of the receiver module was 26 GHz as previously reported in
[4].
We measured an average responsivity over all photodiodes of 0.03 A/W at 1550 nm including the 9 dB intrinsic loss
of the PBS and the 2x4 MMI coupler. Fig. 3 shows the polarization extinction ratio (PER) measured for all eight
photodiodes versus wavelength using the signal input. A PER of >19 dB was obtained for all PDs. A similar result
within 0.4 dB was obtained when using the LO input.
3. DP-QPSK measurement results
OML5.pdf
Fig. 3. PER vs. wavelength in signal path.
The experimental setup for the back-to-back characterization of the coherent receiver module in a DP-QPSK system
is shown in fig 4. At the transmitter, an external cavity laser (ECL) followed by a single-drive IQ modulator
(Fujitsu) is used as signal source. The IQ-modulator is driven by the in-phase and quadrature component of a
28 GBd QPSK signal. The bit sequence consists of 2 14 randomly chosen bits, which are generated by a fieldprogrammable
gate array (FPGA) -based real-time transmitter (Tx). After modulation the single polarization signal
passes a polarization multiplexing stage (PMUX), consisting of a coupler, a delay line and a polarization beam
combiner. On the receive side, a variable optical attenuator (VOA) and an erbium doped fiber amplifier (EDFA)
allow for varying the optical signal-to-noise ratio (OSNR). After passing the preamplifier and a 10 dB coupler for
OSNR evaluation the signal is filtered and boosted by a second EDFA. The average optical signal power launched
into the coherent receiver was -5.5 dBm. The light of a second free-running ECL with about 100 kHz linewidth was
used as LO. After amplification and filtering the average optical LO power launched into the coherent receiver was
14.5 dBm. While the state of polarization of the signal light was arbitrary during the experiments, the LO light was
launched into the slow axis of the receiver’s polarization maintaining input fiber. For the experiment the receiver
module was mounted on an adapter board with coaxial rf-interface. Both, photodiodes and TIAs were operated at
3.5 V. A Tektronix 50 GSa/s real-time oscilloscope with 4 channels and 8 bit nominal resolution was used as
analog-to-digital converter (ADC) to capture and digitize the output voltages of the coherent receiver (XI, XQ and
YI, YQ). The digital post-processing was performed offline in a computer. The post-processing includes: resampling
to an integer number of samples per symbol, correction of the 90° hybrid phase errors and imbalance by using the
Gram-Schmidt orthogonalization [6], intra-scope skew correction and FFT based LO offset frequency correction [7].
After that, the two polarization tributaries are separated and the signal clock is recovered by four FIR filters arranged
in a butterfly structure. The corresponding filter coefficients are adapted using the constant modulus algorithm
Fig. 4. Experimental setup for back-to-back measurements.
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(CMA). After pre-convergence the error criterion is switched to the multi-modulus algorithm (MMA) described in
[8] and finally the equalizer is switched to decision directed mode. After equalization, the bits of the individual
tributaries are decoded and errors are counted. Measurements with 112 Gbit/s DP-QPSK modulation format were
carried out at signal wavelengths of 1530 nm, 1550 nm and 1563 nm. It should be noted that the LO wavelength was
adjusted to the particular signal wavelength to maintain a small frequency difference between signal and LO of a
few MHz. Fig. 5 shows the measured bit-error ratio (BER) vs. OSNR together with a theoretical curve obtained
from [9]. For a BER of 10 -3 an OSNR of 14.6 dB was required at all considered wavelengths. Compared to theory,
this is a penalty of only 1.3 dB. At BER 10 -5 the required OSNR was 18.2 dB at 1550 nm and 1563 nm. However, at
1530 nm wavelength we observed an additional penalty of 0.6 dB at BER 10 -5 which can be attributed partly to
imperfections in the transmitter, filters and in the receiver.
4. Summary
The to-date smallest polarization-diversity coherent receiver module for 100G is presented. The receiver is based on
the hybrid integration of free-space micro-optics and InP PICs. It has a high polarization extinction ratio of >19 dB
over the C-band. The receiver was successfully tested in back-to-back system experiments at 112 Gbit/s DP-QPSK.
Compared to theory, we found a penalty of only 1.3 dB in OSNR sensitivity at BER 10 -3 for three considered Cband
wavelengths.
5. References
OML5.pdf
Fig. 5. BER vs. OSNR (0.1 nm optical bandwidth) at 112 Gbit/s. The insets
show the constellation diagrams of X and Y channels at OSNR = 19 dB.
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paper OMK5, 2009.
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